Copied the O-mode CW/Pulse session on 5125.0 Khz (yesterday 5095.0 KHz was the working frequency). From about 2055Z, I start to hear a serie of ticks: as twitted by Chis Fallen "My exp. uses 60s ON / 60s pulse 10ms w/ 1s inter pulse period (IPP)".

24 July

quoting from Chris Fallen KL3WX "If you are reading this then another radio event of possible interest is
the upcoming Arecibo ionosphere HF heating campaign during 24 to 31
July 2017. The new Arecibo ionosphere HF heater nominally transmits 600
kilowatts net power (100 to 200 megawatts effective radiated power) and
has a unique Cassegrain dual-array antenna design that increases gain of
three crossed dipoles for each band using the signature 1000 ft
spherical dish reflector.[...]. During the upcoming campaign, the Arecibo HF transmitter is limited to two frequencies, 5.125 and 8.175 MHz. Campaign HF transmissions will start at approximately 1600 hours UTC and be active approximately 24 hours per day, with some occasional downtime for maintenance and other activities lasting one or more hours. Generally, the 8.175 MHz transmissions will occur in the daytime when foF2 is expected to exceed that value, between approximately 1830 and 2230 hours UTC. Otherwise the HF transmissions will occur at 5.125 MHz.

These transmissions will be in the vertical direction so this is an excellent opportunity to observe NVIS from a powerful transmitter in Puerto Rico."

Notice that 5125 and 8175 KHz, depending on local foF2, are just estimates and the actual transmitted frequency may be adjusted slightly from those values for various reasons.

I copied one of their transmission on 5095 KHz around 2145 Z on 24 July: the working frequency was comunicated by Chris Fallen in his twitter account: https://twitter.com/ctfallen (follow him if you want tune such these transmission from Arecibo). Although at a first glance the signal appears as "carrier", it consists of O-mode or X-mode polarized CW pulses (Fig. 1):

Fig. 1 - the received signal

These transmissions leave the earth in the vertical direction so this is an excellent opportunity to observe NVIS from a powerful transmitter in Puerto Rico: signals in Europe are probably heard thanks to secondary radiation lobes at 5 MHz (Fig. 2), as depicted in: